The present invention relates to gamma cameras and more specifically to a gamma detection system including a locking mechanism for maintaining two gamma detectors in specific configurations.
Single photon emission computed tomography (SPECT) examinations are carried out by injecting a dilution marker comprising a compound labeled with a radiopharmaceutical into the body of a patient to be examined. A radiopharmaceutical is a substance that emits photons at one or more energy levels. By choosing a compound that will accumulate in an organ to be imaged, compound concentration, and hence radiopharmaceutical concentration, can be substantially limited to the organ to be imaged.
While moving through a patient's blood stream the marker, including the radiopharmaceutical, becomes concentrated in the organ to be imaged. By measuring the intensity of the photons emitted from the organ, organ characteristics, including irregularities, can be identified.
To measure photon intensity, at a prescribed time following marker injection, a planar gamma camera is positioned adjacent the portion of a patient's body that includes the organ to be images. During an imaging period with the camera supported in a single position and the patient remaining as still as possible, the camera detects photon emissions and can create a plan view of the organ corresponding to the camera position.
A gamma camera consists of a collimator, a scintillation crystal and a detector. The collimator typically includes a lead block with tiny holes therethrough which define preferred photon paths. The preferred paths are usually unidirectional and perpendicular to the length of the collimator. The collimator blocks emissions toward the crystal along non-preferred paths.
The scintillation crystal is positioned adjacent the collimator on a side opposite a patient. The crystal absorbs photons that pass through the collimator on a front surface and emits light from a back surface each time a photon is absorbed. The detector comprises a planar arrangement of photomultiplier tubes (PMTS) positioned adjacent the crystal and on a side of the crystal opposite the collimator. Light emitted by the crystal is detected by the PMTs which in turn generate analog intensity signals indicating the precise position of emission impact on the crystal.
A processor receives the PMT signals and digitally stores corresponding information as an M by N array of elements called pixels. The values of M and N are commonly 64 or 128 pixels across each of the two image dimensions. Together the array of pixel information is used by the processor to form an emission image corresponding to the specific camera position.
In addition to the camera and processor, gamma detection systems also include either a stand or a gantry and a patient support table. The stand or gantry supports the camera in one position at a time adjacent a portion of a patient to be imaged and can be used, after data to generate one image has been collected, to move the camera to a second position with respect to the patient, to generate a second image if desired.
Different materials are characterized by different gamma particle attenuation coefficients (i.e. photons are attenuated to different degrees as they pass through different portions of a patient's body). For example, a greater percentage of photons will pass through an inch of bone than will pass through an inch of tissue. Similarly, a greater percentage of photons will pass through the air filled space in a lung or sinus cavity than will pass through a comparable space filled with tissue or bone. In addition, a smaller percentage of photons will pass through four inches of tissue than will pass through one inch of similar tissue.
For this reason, image quality is effected by camera distance from an organ to be imaged. Therefore, whenever images are generated, every effort is taken to position an organ to be imaged as close to the camera as possible by either moving the camera (i.e. the stand or camera on the gantry) or the patient (i.e. the patient table).
Most gamma imaging procedures are used to generate tomographic images and therefore require a plurality of emission images, each image taken by positioning the detector parallel to, and at a different imaging angle about, an imaging axis. In order to produce the best images possible, between each imaging period, the patient may be repositioned with respect to the camera so that the organ to be imaged is as close as possible to the camera. After a plurality of images are generated, a processor is used to compensate for patient repositioning during separate imaging periods. Next, the processor uses the compensated image data to construct pictures of transaxial slices of the body using algorithms and iterative methods that are well known to those skilled in the tomographic imaging art.
Where an organ to be imaged is located on one side of a patient's body, often images will only be generated for the side of the patient's body which includes the organ so that data from the other side of the patient's body which has been attenuated to a greater degree does not reduce the quality of the final images. For example, in the case of heart imaging (i.e. cardiac imaging), typically images will only be generated for 180.degree. through the left hand side of a patient's chest and the other 180.degree. of possible data will be disregarded. In this case, the processor generates the tomographic images using the reduced set of emission data. In addition to increasing the quality of resulting images, limiting imaging to a single side of a patient's body reduces imaging time.
Ideally, given enough time, a gamma camera can provide extremely accurate images for diagnostic purposes. Unfortunately, in reality, there are practical limitations on imaging period length which have to be considered when configuring a detector system and planning imaging sessions. First, imaging hardware and software are relatively expensive and therefore, imaging throughput (i.e. number of imaged patients) must be kept high to justify system costs.
Second, because a patient has to remain nearly completely still during an imaging procedure, prolonged imaging procedures cause patient discomfort. A related problem is that when a patient becomes uncomfortable, the patient often moves. When a patient moves resulting images are distorted. Distorted images can cause medical personnel to incorrectly diagnose healthy tissue as irregular tissue or vice versa. For these reasons any method to reduce the length of imaging periods without reducing accuracy would be advantageous.
One solution to reduce imaging period length is to alter the radiopharmaceutical so that photon emission levels are increased. In this way, data sufficient to form an image could be gathered in relatively less time. Unfortunately this strategy can be dangerous to a patient's health. Typically, in order to minimize patient exposure to radiation, a radiopharmaceutical is chosen which has relatively low photon emission levels. As a result, each emission image requires an appreciable amount of time (e.g. 40 seconds) to generate.
Another solution is to reduce the number of images generated. Unfortunately, this solution reduces image quality which can itself result in diagnostic errors. Typically, to generate sufficient data to form useable tomographic images, at least 64 views equispaced about 360.degree.s surrounding the portion of a patient's body including an organ to be images are required. With 64 views generated by a single camera, an entire imaging process can take longer than 40 minutes to complete. To increase image quality by reducing image granularity, the total number of images can be increased. If the number of images is doubled to 128, required imaging time can be as long as one hour and twenty minutes.
One other solution is to provide more than a single detector so that data for more than one imaging angle can be collected simultaneously. While additional imaging hardware increases system cost, the additional cost of some hardware intensive systems can usually be justified by increased throughput, increased patient comfort and better images due to less patient movement.
For these reasons many systems now employ two gamma cameras positioned around an imaging area to generate two emission images simultaneously. One popular two camera configuration includes two gamma cameras, each centered on a different camera axis that passes through the imaging axis wherein the camera axes are separated by 180.degree. (i.e. the two cameras oppose each other). In this case, 360.degree. imaging can be provided by rotation through 180.degree. and imaging time can be cut in half. Here, for the cost of two cameras, imaging time is reduced by 50%.
In the case of cardiac imaging, two cameras can be configured so that their camera axes form a 90.degree. angle about the imaging axis. In this case, the 180.degree. of imaging data required to generate a tomographic image can be collected by rotation of the cameras through 90.degree..
To accommodate fast 360.degree. and 180.degree. imaging, many gamma camera systems now include two gamma cameras which can be configured in either an opposing position or so that their camera axes define a 90.degree. angle about the imaging axis. To this end, these systems include two motors and associated control hardware and software, one motor for moving each of the two cameras independently of the other camera. Thus, when 360.degree. imaging is required, the cameras are oriented so as to oppose each other and the first and second motors simultaneously move the two cameras about the imaging axis through 180.degree. rotation, stopping at each required imaging angle.
When 180.degree. imaging is required, one of the motors is used to rotate an associated camera about the rotation axis while the other camera is stationary until the camera axes define a 90.degree. angle about the imaging axis. Then, to generate 180.degree. imaging, the two cameras are rotated through 90.degree., one camera collecting 90.degree. of data and the other camera collecting the other 90.degree. of data.
While these two camera systems are versatile and relatively fast, they have a number of shortcomings. First, these systems are hardware intensive. Each system requires two separate motors and associated control hardware, one motor and associated hardware for each of the two cameras. In addition, each system also requires dedicated control hardware and software that can correlate the positions of both cameras during imaging to ensure that the 180.degree. or 90.degree. angle between the camera axes remains constant.
Second, referring to FIG. 1, a system including two cameras 10, 12 configured so that their camera axes define a 90.degree. angle about an imaging axis 11 is illustrated. To reduce stray radiation within an imaging room, gamma cameras 10, 12 typically include a radiation stopping boot 14, 16 which cradles the crystal and detector (not shown) so that photons entering camera collimators 17, 19 are stopped inside the boot. The two cameras 10, 12 are shown mounted to an annular gantry 21 that defines an imaging area 20.
When two cameras 10, 12 are oriented at 90.degree. about a rotation axis 11, the boots 14, 16 interfere at a common edge. Because the boots 14, 16 prohibit crystal placement along the lateral most sections of the cameras, when the boot edges interfere, a dead space 18 that cannot be images by either camera 10, 12 results. If an organ to be imaged is located within dead space 18, imaging information cannot be obtained without moving the patient to a different section of the imaging area 20. While this may be acceptable in some cases, in other cases the increased distance between an organ to be imaged and the cameras will reduce data precision and will effect final image quality. In addition, in some cases where the cameras are supported by a small gantry, it will be impossible to move a patient into a position within the imaging area 20 where an organ to be imaged will not be at least partially within the dead space. In these cases only inferior images generated on the side of the patient opposite the organ will be generated, the usefulness of which would be questionable. Thus, with cameras spaced at 90.degree. about the rotation axis, the effective imaging area 20 is substantially reduced.
For all of the reasons discussed above, it would be advantageous to have a gamma detection system that can quickly collect emission data required to generate precise tomographic images through either 360.degree. or 180.degree. of rotation which is relatively inexpensive.